Circuit SwitchingPacket SwitchingWAN Virtual CircuitsWAN Dialup ServicesWAN Devices

WAN SwitchAccess ServerModemCSU/DSUISDN Terminal Adapter

Introduction to WAN TechnologiesThis chapter introduces the various protocols and technologies used in wide-area network (WAN)environments. Topics summarized here include point-to-point links, circuit switching, packet switching, virtual circuits, dialup services, and WAN devices. Chapters in Part 3, "WAN Technologies," of this book address specific technologies in more detail.What Is a WAN?A WAN is a data communications network that covers a relatively broad geographic area and often uses transmission facilities provided by common carriers, such as telephone companies. WAN technologies function at the lower three layers of the OSI reference model: the physical layer, the data link layer, and the network layer.Figure 3-1illustrates the relationship between the common WAN technologies and the OSI model.

Figure3-1:WAN technologies operate at the lowest levels of the OSI model.